Underground threats in the context of war and military activities refer to objects and tactics employed in attacking an enemy force which make use of the ground as a camouflage for concealing these threats. For example, underground threats can include landmines, buried explosives, booby traps placed within the ground, underground tunnels, covered holes and pits, roadside charges against convoys and the like. Underground threats can also include changes in ground composition which may pose a threat to heavy military equipment and military personnel, such as muddy terrain, swamps, quicksand and the like. With the blurring of the boundary between straight out war and guerilla or terrorist activity, underground threats have increased in number over the years as they are in general easily acquired and built (such as in the case of landmines or explosives) yet difficult to detect. In addition, underground threats are very difficult to neutralize in a military context as underground threats need to be detected in real-time without the knowledge of an expert.
One known method for detecting underground objects is underground imaging using techniques taken from the fields of mineral and oil exploration. In such techniques, a geological survey is taken of an area or region of interest. Based on the geological survey, an image of the ground and what lies beneath can be reconstructed and objects or threats in the ground can be determined. In general, geological surveys are major operations which take a significant amount of time to set up and complete. Also, field experts are usually required to read the geological surveys and interpret the data to determine what objects lie in the ground beneath.
Another known method for detecting underground objects is seismic mapping. In this method, devices known as geophones, which can detect and record seismic responses of the ground over time are positioned in the ground in an area of interest. Geophones are in general inserted into the ground and set up in an array format. One or more seismic sources are then used to generate seismic waves over a period of time in the area of interest. The seismic sources can be manually or hydraulically activated hammers. The seismic waves are substantially reflected and diffracted by objects, open spaces and general differences in ground composition. The geophones which were placed in the ground are synchronized with one another and detect the seismic responses of the area of interest based on the reflections and diffractions of the seismic waves received. Algorithms are then used to extract the underground structure of the area of interest as well as the presence and position of any objects or open spaces in the ground. These algorithms substantially reconstruct a seismic map of the ground under the area of interest. This method is precise and accurate and can be used to detect objects in a significantly large area of interest in a single survey. At the same time, this method is difficult to implement practically in a military context to detect underground objects in real-time as the set up time of installing and placing the geophones in the ground is too lengthy and slow for military use, for example, during a time of war. In addition, setting up an array of geophones for seismic mapping is usually a costly and cumbersome procedure, as the array needs to be checked and calibrated before it can be used to record seismic responses. Such a set up can take days to prepare and fully install, although once prepared, seismic measurements can be taken almost instantaneously.
A further known method for detecting underground objects is ground penetrating radar (herein abbreviated GPR). In GPR, electromagnetic waves in the 1-100 kilohertz (herein abbreviated kHz) range are directed towards an area of interest. In this frequency range, the electromagnetic waves can penetrate the ground up to tens of meters. Reflections from these electromagnetic waves are received and can be used to determine the structure of the ground up to tens of meters below the surface of the ground, including the detection of objects. In general, GPR systems need to be in close proximity to the area of interest, usually within ten meters of the ground. Therefore, an area of interest in which it is suspected that it may contain underground objects must be scanned by a GPR system, which is a procedure that can be time consuming. This method is used in military contexts although it may hamper the mobility of the army units which use and require such systems.
Another known system for detecting underground objects, such as landmines, is the laser Doppler vibrometer (herein abbreviated LDV). LDV systems are based on interferometry and substantially measure Doppler shifts between a laser beam aimed at a target surface and a reference beam. LDV systems are very sensitive and can detect nanometer size vibrations on a target surface yet are ineffective by themselves in determining a seismic map of an area of interest, especially of the volume beneath a target surface. LDV systems are ineffective in such tasks since they are very sensitive to turbulence and have a fixed sensitivity with respect to distance. LDV systems have been combined with acoustic systems in which strong sound waves are directed towards a region of interest, thereby causing small vibrations in the ground in the region of interest. An LDV system is then used to measure differences in frequency of the laser beam directed at the region of interest and a reference beam, thereby generating a seismic map. Such systems are limited though in detecting underground objects as the seismic information extracted from such systems is not as full as the seismic information which can be extracted from geophones.
Other systems for remotely detecting underground objects are known in the art. U.S. Pat. No. 6,809,991 issued to Pepper, et al., entitled “Method and apparatus for detecting hidden features disposed in an opaque environment,” is directed to a system for remotely locating and identifying features disposed within an opaque environment, such as a landmine buried under the surface of the ground. The system includes two laser sources, a vibration sensor module and a signal processing unit. One laser source produces a modulated exciter beam with the other being a probe beam. The signal processing unit receives signal information from the vibration sensor module and controls the modulation of the exciter beam.
The exciter laser periodically emits a modulated beam which, upon absorption in the ground, generates an acoustic wave which propagates along the surface of the ground as well as in the subsurface. The acoustic wave is produced through thermo-elastic and/or ablative effects. The acoustic modes within the ground are scattered due to inhomogeneities such as buried objects. The acoustic spectrum generated in the ground substantially replicates the modulation format of the exciter laser. A small portion of the scattered waves travels back to the surface resulting in small but detectable vibrations. The probe laser detects these vibrations as the laser beam impinges on the surface of the ground and a small portion of the laser beam is reflected back by the surface towards the system. The vibrations of the surface are superimposed on the reflected beam. The reflected beam is provided to the vibration sensor module which converts the light wave into an electric signal. The electric signal is supplied to the signal processing unit. The information in the electric signal is representative of the vibrations at the surface which in turn is representative of a buried object. The signal processing unit analyzes the signal and determines what type of object is buried in the ground by comparing the information in the received signals to a set of predetermined data patterns. The predetermined data patterns correspond to a variety of different objects which might be encountered, such as a landmine, a rock, a tree root and so forth.
The processor selects an object and changes the characteristics of the exciter laser beam in order to adjust the generated acoustic waves so as to achieve acoustic modes that best couple with the selected object. By analyzing the information received from the vibration sensor module after the change in characteristics, the processor verifies its selection. The processor may reject its selection and try various other characteristics of the exciter laser beam in order to determine what object is buried in the ground.
U.S. Patent Application Publication No. 2003/0189708 to Chang, entitled “Antitank mine detection system for armored vehicle” is directed to a system for armored vehicles for remotely detecting antitank mines. The system includes an armored vehicle for carrying the optical and electronic components of the system. The armored vehicle is also used as an exciter for seismic waves. The system also includes an optical source body disposed on the front end of the armored vehicle, a sensor disposed on the side of the optical source body and a controller which controls the radiation from the optical source body as well as the speed of the armored vehicle. The controller includes a data processing part for converting an image received by the sensor to an electric signal and for processing it.
The body of the armored vehicle, along with its load, serves as a source for seismic wave motion which is distorted due to the presence of an antitank mine. The distortion is located by the system by measuring fluctuations of the ground. The optical source produces two laser beams, an object beam which is directed to the ground and a reference beam. Part of the object beam is reflected back towards the sensor. The reflected object beam and the reference beam are collected by the sensor, thereby obtaining an interference speckle image. The image is then processed in real-time by the data processing part which detects the point where the wave motion is distorted. By comparing the data collected from the interference speckle image with existing data stored therein, the processor determines whether the object distorting the wave motion is an antitank mine or a rock.
An article published in the Proceedings of the SPIE, vol. 5794 (June 2005), pp. 624-631, by Aranchuk, et al., entitled “Multi-beam laser Doppler vibrometry for acoustic landmine detection using airborne and mechanically-coupled vibration,” describes a system for detecting buried landmines using Doppler interferometry and acoustic-to-seismic coupling. The system includes a multi-beam laser Doppler vibrometer (LDV), a phase-lock loop demodulator, a computer employed for signal processing, and either airborne sound (i.e., specially designed loudspeakers) or mechanical shakers to excite vibrations of the ground.
A vibration is excited in the ground. The LDV produces a laser beam which is split into 16 object beams and 16 reference beams. The 16 object beams are focused onto the ground along a line. A portion of the object beams which are scattered back is combined with the reference beams whose frequency is shifted by 100 kHz, thereby producing 16 frequency modulated signals with a 100 kHz carrier frequency. The frequency deviation, due to the Doppler Effect, of each signal is proportional to the velocity of the ground at the point of measurement. The output signals of the LDV are demodulated by the phase-lock loop. Each of the 16 output signals of the phase-lock loop is then digitized by the computer which calculates the velocity spectrum of each beam. All of the beams can be moved forward (that is in a direction perpendicular to the line formed by the beams) by using a rotating mirror so that an area segment can be scanned to generate a velocity image over the scanned area. Landmines buried in this area can be located by examining the ground velocity image.
U.S. Pat. No. 7,583,387 issued to Meldahl, et al., entitled “Seismic exploration” is directed to a system and method for seismic exploration and seismic imaging by using a moving laser interferometer, in particular for use in submarine seismic exploration. The system comprises an interferometer which includes a source of coherent object light, a source producing a reference beam which is coherent with the object beam, and a detector or array of detectors. Additionally, the method may include a step of generating a seismic event such that the system can detect the response to the event.
An object beam is sent from the interferometer towards an inspected surface, e.g. the sea bed. Part of the object beam is reflected back up towards the interferometer where it is combined with a reference beam to illuminate a detector. In the case where an array or a line of detectors is used the reference beam, or a set of combined spatially distributed reference beams, must cover the whole array. The combination of the object beam and the reference beam creates an interference pattern that is detected by the detector. The signals from all of the detectors are digitized and fed to a processor which calculates the movement of the inspected surface.
The object beam sent from the interferometer is first expanded and then arranged to converge at a point which is approximately the same distance beyond the measured surface as the surface is spaced from the beam source. This feature and other means, such as modulating the reference beam, allow the system to measure the movement of the surface while in motion, for example by being towed by a ship. The speed of motion of the interferometer, the sampling rate of the detectors and the size of the area illuminated by the object beam are arranged so that sequential areas of the surface overlap.